The present invention relates to a combustor for burning fuel in compressed air. More specifically, the present invention relates to a low NOx combustor for a gas turbine.
In a gas turbine, fuel is burned in compressed air, produced by a compressor, in one or more combustors. Traditionally, such combustors had a primary combustion zone in which an approximately stoichiometric mixture of fuel and air was formed and burned in a diffusion type combustion process. Additional air was introduced into the combustor downstream of the primary combustion zone. Although the overall fuel/air ratio was considerably less than stoichiometric, the fuel/air mixture was readily ignited at start-up and good flame stability was achieved over a wide range in firing temperatures due to the locally richer nature of the fuel/air mixture in the primary combustion zone.
Unfortunately, use of such approximately stoichiometric fuel/air mixtures resulted in very high temperatures in the primary combustion zone. Such high temperatures promoted the formation of oxides of nitrogen ("NOx"), considered an atmospheric pollutant. It is known that combustion at lean fuel/air ratios reduces NOx formation. However, achieving such lean mixtures requires that the fuel be widely distributed and very well mixed into the combustion air. This can be accomplished by introducing the fuel into both primary and secondary annular air inlets using, in the case of gas fuel, fuel spray tubes distributed around the circumference of the annulus.
It has been found that mixing of the fuel and air is enhanced by using separate passages to divide the air in the primary air inlet into two streams. Radial swirlers, comprised of a number of swirl vanes distributed around the circumference of these passages, impart a swirl angle to the air that aids in the mixing of the fuel and air. The swirlers in each primary inlet passage are opposite handed so that the air exiting from the pre-mixing zone has little net swirl angle. Such a combustor is disclosed in "Industrial RB211 Dry Low Emission Combustion" by J. Willis et al., American Society of Mechanical Engineers (May 1993).
Unfortunately, such combustors suffer from a variety of drawbacks. First, the swirl vanes are integrally cast into a primary air inlet assembly, making it impossible to change the swirl angle once the combustor has been built. This makes it difficult to optimize the swirl conditions since it is not possible for the combustor designer to predict in advance the specific swirl angle that should be imparted to the air in order to achieve optimum results at a minimum pressure drop. Second, there is no capability of burning liquid fuel in such combustors since fuel spray tubes are relied upon exclusively to introduce fuel. Third, the fuel spray tubes that introduce fuel into the secondary air inlet passage are oriented axially and located upstream of the passage's inlet. This results in the failure of a portion of the fuel to enter the secondary air inlet passage, causing fouling and contamination of the combustor components exposed to the fuel. Fourth, the inner liner enclosing the primary combustion zone is subject to over-heating and deterioration, especially at its outlet edge.
It is therefore desirable to provide a gas turbine combustor having adjustable swirl vanes, dual fuel capability, accurate introduction of fuel into the secondary air inlet passage and adequate cooling of the liner that encloses the combustion zone.